Isolation and characterization of a new DUR3-like gene, PyDUR3.3, from the marine macroalga Pyropia yezoensis (Rhodophyta)

Abstract

DUR3 proteins mediate high-affinity transport of exogenous and endogenous urea. Although two DUR3-like genes (PyDUR3.1/3.2) have been identified in Pyropia yezoensis, a BLAST search using these sequences against the P. yezoensis EST database suggested the existence of another DUR3-like gene (PyDUR3.3). In this study, the PyDUR3.3 gene was isolated and characterized and compared to PyDUR3.1/3.2 genes. The predicted length of PyDUR3.3 was 679 amino acids, which included 15 transmembrane domains. An amino acid sequence alignment of algal, plant, and yeast DUR3 proteins showed that PyDUR3.3 was more similar to PyDUR3.2 than to other DUR3 proteins, including PyDUR3.1. Exon–intron structures of PyDUR3.3 and PyDUR3.2 were also closely related to each other, which clearly differed from that of PyDUR3.1. Expression analysis showed that PyDUR3.3 mRNA levels were extremely high in sporophytes, regardless of the nutrient condition, compared to gametophytes. On the other hand, expression of PyDUR3.2 and PyDUR3.1 was high in the gametophytes and sporophytes, respectively, and was induced by nutrient starvation. These results suggest that expression of PyDUR3.3/3.2/3.1 depends on the life history phase as well as the nutrient conditions, and that PyDUR3.3 and PyDUR3.2 are paralogues specifically differentiated in function and life history phase.